Silanes are commonly employed as cross-linkers for the production of PEX-b articles (silane cross-linked polyethylene) such as pipe, wire coatings, insulation jacketing for voltage cables, insulation foams, and heat shrinkable products. The silane is typically used in conjunction with a peroxide, which facilitates grafting of the silane onto the cross-linking polymer.
Traditional PEX-b production technologies include the two-step “Sioplas” process and the single-step “Monosil” process. The two methods each graft vinylsilane, such as vinyltrimethoxy silane, onto polyethylene and then moisture cross-link silane groups into three dimensional cross-linked polyethylene. The grafting reaction is typically performed in a single screw extruder, while the hydrolysis/condensation reaction can be done under a variety of conditions, including exposure to moisture under ambient conditions, exposure to hot water via submersion of the grafted resin, or exposure to steam. In the production of potable water pipes, for example, hot water may be circulated through the extruded pipe to complete the crosslinking.
Often, when silane is grafted onto polyethylene chains, the side chains can contain multiple silanes due to homopolymerization of vinylsilane. Theoretically, one silane group per graft chain would be sufficient for cross-linking, i.e., the use of expensive silane monomers may not be efficient with typical current methods. In addition, during moisture cross-linking, each of the silanes in the same side chain can participate in cross-linking. Thus, cross-links may not be evenly distributed all across the polyethylene materials. This reaction is shown below:

In view of the above, it would be desirable to more efficiently use silane monomers and more evenly distribute the cross-linking in PEX-b materials, thereby increasing long-term performance of PEX-b articles.